It is of great interest to determine the ion selectivities of molecular ionophores which are known to function as ion carriers across lipid barriers. However in order to determine the ion selectivities from transport experiments, most analyses involve ad hoc assumptions. Thus transport ion selectivities (versus equilibrium ion selectivities) of ionophores have not been measured directly. This project is to measure the transport ion selectivities and at the same time study the transport kinetics of ionophores by using simplest possible model systems such that only minimum assumptions are needed in data analyses. Our typical model system is a U-tube whose bottom is filled with organic solvent (in which ionophores are dissolved) separating two different ionic solutions in side arms. In addition to measurement of ion selectivities under transport condition, we will investigate the possible dependence of these selectivities on thermodynamic and transport parameters such as dielectric constant of organic solvents, transport rate, and anions used in cation transport systems. We will also test the kinetic theory of ion transport which we proposed previously. In short we study the intrinsic transport properties of ionophores. With such knowledge, one can use them as molecular probes to investigate cellular ion transport.